Two-stroke engines are conventionally used as the driving engines of portable power tools, such as portable trimmers for trees and plants and backpack power tools, carried by the operators themselves or on the backs of the operators. However, for example, as awareness of environmental issues grows and emission regulations become more stringent, there is an increasing need for replacement of two-stroke engines used as driving sources with four-stroke engines.
However, the numbers of required components of four-stroke engines are greater than those of two-stroke engines, and therefore the weights of the four-stroke engines tend to be greater than those of the two-stroke engines. A portable power tool, in particular, is used on the premise that the operator carries the tool during operation, and therefore there is a demand for a weight reduction of the engine.
A four-stroke engine provided with a lubrication system is developed (see Patent Document 1). In this lubrication system, a pump for lubrication is not separately provided, and pressure changes in a crank chamber are utilized to circulate oil. This lubrication system includes a check valve disposed in the bottom portion of the crank chamber. This check valve opens when a positive pressure is created in the crank chamber, and oil or oil mist in the crank chamber is delivered to an oil reservoir. The oil or oil mist passes through a communication tube provided in the oil reservoir and is supplied to a valve-operating chamber and a valve-operating unit for driving the valves, and a sufficient amount of oil is thereby supplied to the valve-operating chamber and the valve-operating unit. A communication hole is provided in the crank chamber so as to communicate the crank chamber with the valve-operating chamber when a piston moves upward. The oil accumulated in the valve-operating chamber is returned to the crank chamber through the communication hole when the communication hole opens.
If blow-by gas enters a lubrication path for the lubricating oil, the concentration of the blow-by gas in the lubrication path increases. In this state, if the blow-by gas remains mixed with the oil, the oil is gradually degraded, and this adversely affects the lubrication of driving components. Therefore, measures are generally taken to discharge the blow-by gas in the lubrication path into a combustion chamber to prevent early deterioration of the oil. A structure for allowing the valve-operating chamber and the combustion chamber to communicate with each other through an air cleaner is often used, as also described in Patent Document 1.
Patent Document 2 proposes a lubrication system for a four-stroke engine. In this lubrication system, a pump for lubrication is not separately provided, and pressure changes in a crank chamber are utilized to circulate oil. In this lubrication system, a negative pressure created in the crank chamber is utilized to supply oil mist generated in an oil tank to the crank chamber through a first oil passage that is drilled in a crankshaft and communicates the oil tank with the crank chamber, and the crankshaft and components therearound are thereby lubricated. The floating oil mist generated in the oil tank is delivered, by utilizing a positive pressure created in the crank chamber, to a power transmission mechanism (including an intake valve and an exhaust valve) in a first valve-operating chamber and a cam mechanism in a second valve-operating chamber, which are disposed above the oil tank when the engine is upright, and these driving components are thereby lubricated.
A partition plate is disposed inside a head cover that forms the second valve-operating chamber. The partition plate partitions the space inside the head cover into an upper section serving as a breather chamber and a lower section serving as the second valve-operating chamber. The breather chamber is in communication with the second valve-operating chamber through a communication part that is opened in the second valve-operating chamber. A box-shaped partition member is welded to the partition plate, and an oil collection chamber is formed between the partition plate and the partition member. Suction tubes extending toward the power transmission mechanism in the second valve-operating chamber are provided in the partition plate, and suction tubes extending toward the ceiling surface of the head cover are provided in the partition member. A conduit tube that is in communication with the oil collection chamber and protrudes toward the second valve-operating chamber is provided in the partition plate. The conduit tube is in communication with the crank chamber.
In this lubrication system, when a negative pressure is created in the crank chamber as the crankshaft rotates to move a piston, a negative pressure is also formed in the oil collection chamber through the conduit tube. Therefore, the oil accumulated in the second valve-operating chamber or the breather chamber is sucked through the suction tubes and is returned to the crank chamber.